Merchants may conduct transactions for items and services with customers. To conduct a transaction with a customer, a merchant can use a point-of-sale (POS) device to receive payment from the customer, such as in the form of a payment instrument. The merchant can then use the POS device to authorize the payment instrument for a cost of the transaction. For instance, the POS device can send a request to a payment service to authorize the payment instrument for the cost of the transaction. In response, the POS device can receive a message indicating whether the payment instrument is authorized for the transaction.